Catalysts having both a hydrogenation-dehydrogenation function and an isomerization/cracking function (“dual-function” catalysts) are used widely in many applications, particularly in the petroleum and petrochemical industry, to accelerate a wide spectrum of hydrocarbon-conversion reactions. The isomerization/cracking function generally relates to a material of the porous, adsorptive, refractory-oxide type containing an acid function. Typically, this material may be utilized as a support or carrier. The hydrogenation-dehydrogenation function is primarily contributed by a metal component (e.g., Group VIII metals) that is combined with the support.
It is of critical importance that a dual-function catalyst exhibit the capability both to initially perform its specified functions efficiently and to perform them satisfactorily for prolonged periods of time. The parameters used in the art to measure how well a particular catalyst performs its intended functions in a particular hydrocarbon reaction environment are activity, selectivity and stability. In a reforming environment, these parameters are defined as follows:
Activity is a measure of the ability of the catalyst to convert hydrocarbon reactants to products at a designated severity level representing a combination of reaction conditions: temperature, pressure, contact time, and hydrogen partial pressure. Selectivity refers to the percentage yield of a desired product from a given feedstock at a particular activity level. Stability refers to the rate of change of activity or selectivity per unit of time or of feedstock processed. Activity stability generally is measured as the rate of change of operating temperature per unit of time/feedstock to achieve a given product, with a lower rate of change corresponding to better activity stability.
One process that often employs a dual-function catalyst is catalytic naphtha reforming. Reforming comprises a variety of reaction sequences, including dehydrogenation of cyclohexanes to aromatics, dehydroisomerization of alkylcyclopentanes to aromatics, dehydrocyclization of an acyclic hydrocarbon to aromatics, hydrocracking of paraffins to light products boiling outside the gasoline range, dealkylation of alkylbenzenes and isomerization of paraffins. Some of the reactions occurring during reforming, such as hydrocracking which produces light paraffin gases, are undesirable as they can have a deleterious effect on the yield of a desired product. Improvements in catalytic reforming technology thus are targeted toward enhancing those reactions effecting a higher yield of a desired product.
In some refineries configured for petrochemical production, it may be desirable to carry out additional processing to maximize the yield of valuable xylenes from the aromatic gasoline produced in the reforming process. The xylene isomers are produced in large volumes from petroleum as feedstocks for a variety of important industrial chemicals. Orthoxylene is used to produce phthalic anhydride, which has high-volume but mature markets. Metaxylene is used in lesser but growing volumes for such products as plasticizers, azo dyes and wood preservers. However, the most important of the xylene isomers is paraxylene, the principal feedstock for polyester which continues to enjoy a high growth rate from a large base demand. In addition, often present in xylene mixtures is ethylbenzene, which is occasionally recovered for styrene production, but usually is considered a less desirable component of C8 aromatics.
The xylenes are not directly recovered from petroleum by the fractionation of naphtha in sufficient volume to meet demand nor in a high enough purity; thus conversion of other hydrocarbons is necessary to increase the purity and yield of the xylenes. For straight run naphtha feedstocks, which may be naphtha distilled out of crude oil, it is necessary to utilize high severity reforming with inter-reactor reheat to convert large amounts of paraffins, such as from about 40 to about 70 weight percent, and having about 30 to about 60% total cyclic content, to the desired xylenes and/or benzene. Moreover, the large amount of non-aromatic content remaining in the reformed naphtha requires substantial subsequent processing to remove the non-aromatics and to transalkylate the aromatics to benzene and xylene.
While the aforementioned dual-function catalysts are capable of catalyzing the dehydrocyclization of paraffins to aromatics such as para-xylene, there is always a trade-off where higher acidity catalysts have more activity but also have reduced selectivity due to increased hydrocracked products, particularly propanes and butanes. Therefore what is needed is a way to eliminate this trade-off where higher selectivity does not come at the cost of lower activity.